Glad you found the problem !
I was going to post the the wheel cylinder is froze.
This problem happened on my 87 Sammy but the other way, I HAD NO LEFT REAR BRAKE !!
which wasn't noticed until a spring trail event and I was always getting turned sideways on down hill de-sends.
The problem I believe is that the original wheel cylinder is aluminum, the pistons are steel and good old electrolosis
welds the two together during inactive use ie; winter layover.
CAPT